1. Field of the Invention
The present invention relates to a method of determining exposure conditions for an exposure device for manufacturing semiconductor devices or liquid crystal display devices or the like. More particular, the present invention relates to a measuring method capable of preferably determining the focusing condition of a projection type exposure device (an aligner or a stepper or the like).
2. Related Background Art
In a photolithography process necessary to manufacture semiconductor devices, a circuit pattern written on a glass original plate, which is usually called a "reticle", is printed on a resist layer applied on a wafer by a predetermined thickness (about 1 to 2 .mu.m). Then, the resist layer which has not been removed by a development process is arranged to serve as a mask so that the wafer is subjected to a variety of processes including photoetching. In particular, in the exposure (printing)process, a reduction projection type exposure device (stepper) of a step and repeat type is widely used to serve as a device to transfer the reticle pattern to the wafer while maintaining a high resolution.
A stepper of the type described above is arranged to expose the reticle pattern onto the wafer (the resist layer) via a projection lens after the reticle and the wafer have been accurately aligned to each other. In order to meet a recent desire of manufacturing a densely integrated and precise semiconductor device, the wave length of exposure light has been shortened from 436 nm (g-ray) to 365 nm (i-ray) or to 248 nm (KrF Excimer laser). Furthermore, a projection lens having a large N.A. (Numerical Aperture) has been developed.
At present, there has been realized a practical stepper for manufacturing semiconductor devices which is provided with a projection lens the exposure wave length of which is 436 nm (g-ray), the N.A. of which is 0.54 and the reduction ratio of which is 1/5. Furthermore, it is capable of maintaining the resolving power of a minimum line width of 0.65 .mu.m (3.25 .mu.m on the reticle) in the overall region (15 mm.times.15 mm) of the exposure field. Furthermore, since the size of the chip has been enlarged because of the realized high integration, the exposure field also is enlarged up to about 20.times.20 mm.
A projection lens having a large N.A. inevitably has a small depth of focus and encounters a problem in that a so-called film reduction occurs, the film reduction being a problem of a type in which a portion of the resist image is removed due to the development processing depending upon the deviation quantity (the defocus amount) of the surface of the wafer with respect to the best focus position at the exposure. As a result, the image contrast of the resist image will undesirably be deteriorated. If the film reduction occurs in the direction of the thickness of the resist film, the thickness of the resist film will be reduced after the development. Therefore, a disadvantage takes place in the next etching in which the above-described resist image is arranged to server as the mask. That is, the basic layer will be removed (subjected to the etching) even if there is the resist image (mask). In the case where the film reduction takes place in the lengthwise direction of the resist image, the two end portions of the resist image will be removed, causing the length of the pattern to undesirably be reduced.
Since this causes the quality of the manufactured device to be deteriorated, it is a critical factor to know the defocus amount which will not cause the film reduction to occur. That is, in order to obtain a resist image exhibiting high image contrast in the wafer process, that is, in order to obtain a device (semiconductor device) capable of meeting the desired characteristics, it is a critical factor to know the focus range (the depth of focus of a projection lens relating to the film reduction) which will not cause the film reduction to occur, by measuring the focus position on the infocus side (adjacent to the projection lens with respect to the best focus position) and the outfocus side (adjacent to the wafer).
Hitherto, in order to know the focus range which will not cause the film reduction to occur, a method has been employed in which the wafer is developed after a trial printing on to the wafer has been performed. Then, the state of the formed resist image is observed by a scanning type electronic microscope (SEM) or an optical microscope. Specifically, a line and space (L/S) pattern is used in which linear patterns (bar patterns) each of which has a predetermined width are arranged at predetermined pitches. Furthermore, only the focus condition is successively changed by a predetermined quantity (for example, 0.2 .mu.m) under the most suitable quantity of exposure (exposure time) so that it is successively exposed onto the wafer. Thus, the resist image of the L/S pattern formed in the shot region (which corresponds to the exposure field of the projection lens) on the wafer is observed by an ITV camera by using the SEM or the optical microscope after the development. In a particular case in which whether or not the thickness of the resist image is reduced in the direction of the thickness is observed by the SEM, there arises a necessity of making a sample the cross section of which is to be observed in such a manner that the sample (the resist image) is cut in a direction (in the direction of the short side) substantially perpendicular to the lengthwise direction. Then, the position on the in and the outfocus side in which the film reduction has not taken place are detected depending upon the state (the film thickness and length of the pattern and the like) in which the resist image is formed so as to make the focus range thus determined to be the depth of focus of the projection lens relating to the film reduction.
However, the above-described conventional technology raises a problem in that the above-described measurement operation must be repeatedly performed in order to measure the focus ranges in the overall region of the exposure field of the projection lens, for example, the focus range of each of a plurality of points at the field center and around the same. Furthermore, the state in which the resist image has been formed must be observed by using the optical microscope or an exclusive measuring device such as the SEM. As a result, the processing speed cannot be raised satisfactorily.
In addition, in the case where the resist image is observed through the ITV camera by using an optical microscope, the operation for adjusting the focus of the microscope must be performed precisely, causing the operator to perform an excessively exhausting work. On the other hand, in the case where the exclusive measuring device such as the SEM is used, the resist image can be observed relatively precisely. However, the wafer (formed by cutting so as to have its cross section observed) serving as the sample must be placed in a high vacuum chamber before the resist image is observed. Therefore, it takes an excessively long time to manufacture the sample the cross section of which is to be observed and the operation for handling the exhaust system, causing a problem to arise in that the throughput of the measurement is too low.
In particular, a projection lens having a wide exposure field raises a necessity in that the focus range resolvable in the overall region of the exposure field without the film reduction is easily measured. However, the projection lens of the type described above raises a problem in that the number of the points to be measured in the exposure field is increased (10 to 20 points). Therefore, it has been impossible in terms of labor and time to measure the focus range in the overall region of the exposure field without the film reduction.